1. Field of the Invention
This invention relates to multi-component systems, especially ceramic-metal composites and to methods of producing these systems, especially such composites. The invention provides a technique for redistributing one material, such as a metal, evenly throughout another material, such as a ceramic. This technique permits fabrication of materials, such as composites, that were formerly unattainable or attainable only at great cost.
2. Description of Related Art
Ceramic materials may be combined with metals to form composites having exceptional hardness, strength and fracture toughness. These materials are useful for cutting tools, structural materials, and armor.
Preferably, the composites are at least 50% ceramic material, and obtaining satisfactory metal distribution throughout a primarily ceramic body has proven very difficult. The metal distribution is important in adding toughness and in obtaining void-free densification.
In general, processes for impregnating a ceramic material with metal have followed two pathways. In the first pathway, a ceramic body is preformed and then coated with a metal. The metal is then driven into the ceramic body using various techniques. The second pathway involves forming a "green" body out of a mixture of ceramic and metal and then attempting to redistribute the two components within the body
Various prior art techniques for either driving the metal into the ceramic or redistributing the metal in a ceramic-metal composite include: hot pressing, hot isostatic pressing and explosive compaction. Each of these techniques, however, is dependent upon the quality of metal distribution between the particles of the ceramic phase. Thus the techniques are limited by the type of ceramic-metal composites that can be formed.
Another technique for preparing a ceramic-metal composite involves heating the composite to a temperature higher than that of the melting point of the metal to encourage metal migration in the composite due to capillary action and surface tension. Often, this procedure is carried out in a vacuum or in the presence of an inert gas to ensure against chemical reaction or contamination of the liquid phase. These processes are exemplified by U.S. Pat. No. 4,605,440 to Halverson et al. (Halverson I) and U.S. Pat. No. 4,718,941 to Halverson et al. (Halverson II).
In Halverson I, a ceramic-metal "green body" is formed and then heated to a temperature from about 1050.degree. C. to about 1250.degree. C. for 2 to 10 minutes in an inert atmosphere or vacuum to obtain "wetting" of the ceramic by the metal. Halverson I is careful to point out that mass transfer between the ceramic and metal takes place during this process, and warns that prolonged heating affects the ultimate product due to inter-phase contamination.
In Halverson II, a ceramic "sponge" is impregnated with metal after preforming and chemical treatment to change the surface chemistry of the ceramic to permit wetting by the molten metal. As in Halverson I, impregnation takes place using capillary action and surface tension.
Unfortunately, the molten metal wetting technique of the Halverson patents is limited to those ceramic-metal systems that do not have either a high metal melting point or unacceptable wetting properties. What is needed in the art is a technique that can both provide for a broad range of ceramic-metal composites, avoid extensive phase intermingling during redistribution and provide a uniform distribution of metals within a ceramic-metal composite.